Technical Field
The present disclosure relates to an exhaust gas purification apparatus for an internal combustion engine.
Description of the Related Art
There are internal combustion engines provided with a catalyst for selective catalytic reduction of NOx (which will be sometimes simply referred to as “NOx catalyst” hereinafter) for reducing NOx in the exhaust gas provided in an exhaust passage. A typical NOx catalyst may include at least one kind of active component such as Fe or Cu having the function of selectively reducing NOx, which is carried in micropores of zeolite by ion exchange.
With the NOx catalyst, NOx in the exhaust gas may be selectively reduced with supply of a reducing agent such as ammonia. When the catalyst temperature of the NOx catalyst is low, satisfactory NOx removal efficiency tends not to be achieved. As a countermeasure, PTL 1 discloses a NOx catalyst constructed in a two-layer structure including a lower catalyst layer and an upper catalyst layer with the lower catalyst layer having oxidation function exercised by noble metal and the upper layer not having oxidation function but having only reduction function. PTL 1 describes oxidation of NO (nitrogen monoxide) into NO2 by the lower catalyst layer and the use of the resultant NO2 for reduction of NOx by the upper catalyst layer with ammonia.